1. Field of the Invention
This present invention relates to a sealing device for a high-pressure vessel, and especially to a sealing device for a surge absorption device, mainly in a high-pressure fuel pump such as is used in a cylinder-injected engine, capable of reducing surge amplitude and thus enabling stabilization of the amount of fuel injected and stabilization of the engine cycle.
2. Description of the Related Art
Diesel engines are the most widely known of the so-called xe2x80x9ccylinder-injectedxe2x80x9d or xe2x80x9cdirect injection enginesxe2x80x9d, engines in which fuel is injected into the engine cylinder, but in recent years cylinder-injected spark ignition engines (gasoline engines) have also been proposed. Cylinder-injected engines of this kind demand that fuel pressure surges be minimized to maintain sufficiently high fuel injection pressure and ensure stable injection. To this end, compact single-cylinder high-pressure fuel pumps have been proposed which are of simple construction and inexpensive to manufacture. However, because there is only one plunger in the single-cylinder system, there are surges of quite some amplitude in the pressure of the fuel discharged, and so surge absorption devices with metal bellows or diaphragms have been proposed to absorb these surges.
FIG. 10 shows a high-pressure fuel supply system provided with a high-pressure accumulator which is a good example of a surge absorption device to which the sealing device of the present invention can be applied. In FIG. 10, a delivery pipe 1, which is a fuel injection apparatus, is provided with a plurality of injectors 1a corresponding to the number of engine cylinders, which are not shown. A high-pressure fuel pump assembly 200 provided with a high-pressure fuel pump 3 is disposed between the delivery pipe 1 and a fuel tank 2. The delivery pipe 1 and the high-pressure fuel pump 3 are connected by a high-pressure fuel passage 4 and the high-pressure fuel pump 3 and the fuel tank 2 are connected by a low-pressure fuel passage 5. Together, the high-pressure fuel passage 4 and the low-pressure fuel passage 5 compose a fuel passage connecting the delivery pipe 1 to the fuel tank 2. A filter 6 is disposed in the fuel intake of the high-pressure fuel pump 3. A check valve 7 is disposed on the fuel discharge side of the high-pressure fuel pump 3. A drain 8 attached to the high-pressure fuel pump 3 returns to the fuel tank 2.
A low-pressure fuel pump 10 is disposed at the end of the low-pressure fuel passage 5 close to the fuel tank 2. A filter 11 is disposed in the fuel intake of the low-pressure fuel pump 10. A check valve 12 is disposed in the low-pressure fuel passage 5 on the fuel discharge side of the low-pressure fuel pump 10. A low-pressure regulator 14 is disposed in the low-pressure fuel passage 5 between the high-pressure fuel pump 3 and the low-pressure fuel pump 10. A filter 15 is disposed in the fuel intake of the low-pressure regulator 14. A drain 16 attached to the low-pressure regulator 14 returns to the fuel tank 2.
The high-pressure fuel pump 3 increases the pressure of the fuel supplied to it by the low-pressure fuel passage 5 and discharges it to the delivery pipe 1. A dumper 30 is disposed on the low-pressure fuel passage 5 side of the high-pressure fuel pump 3, i.e., the low-pressure side. A high-pressure accumulator 70 and a high-pressure regulator 32 are disposed on the high-pressure side of the high-pressure fuel pump 3. A drain 33 attached to the high-pressure regulator 32 returns to the fuel input side of the high-pressure fuel pump 3.
FIG. 11 is a cross-section showing details of the high-pressure fuel pump assembly 200 when fully assembled, comprising the high-pressure fuel pump 3, dumper 30, high-pressure accumulator 70, high-pressure regulator 32, filter 6, and check valve 7. In FIG. 11, a recess portion 40c is formed in the casing 40 on the right-hand side of the diagram, and the high-pressure accumulator 70 is secured to the recess portion 40c. A discharge passage 4b which communicates with a discharge passage 4a is formed as a recess in the bottom of the recess portion 40c. 
FIG. 12 is a cross-section showing details of the high-pressure accumulator 70, which is a surge absorption device to which the sealing device of the present invention can be applied. The high-pressure accumulator 70 is provided with a case 85, which is a high-pressure vessel roughly the shape of a thick disk, a flexible disk-shaped metal diaphragm 86, supported by and sealed against the case 85 around its perimeter portion so that together they form a high-pressure chamber 71, and a disk-shaped plate 89, which is a stopper defining the limit of deformation of the diaphragm 86.
The case 85 has a comparatively thin perimeter portion 72, which supports and seals the outer perimeter portion of the diaphragm 86 by a sealing weld, and a comparatively thick central portion 73, in which the high-pressure chamber 71 is formed. A male thread 91 is formed on the cylindrical outer surface of the peripheral portion 72, and a comparatively shallow saucer-shaped recess portion 74, which gradually deepens from the perimeter portion towards the central portion in a smooth curve to allow the diaphragm 86 to deform towards the high-pressure chamber 71, is formed in the portion in close contact with the diaphragm 86. An approximately-cylindrical recess portion 75, which communicates with the shallow saucer-shaped recess portion 74 at the central portion, is formed in the central portion 73 and, together with the saucer-shaped recess portion 74, forms the high-pressure chamber 71.
A gas charge inlet 84 of circular cross-section about its central axis is formed in the ceiling portion of the high-pressure chamber 71 to introduce high-pressure gas to the high-pressure chamber 71 of the case 85 and seal it in, and a special thread member 87 is disposed therein as a sealing device to seal the gas charge inlet 84. The gas charge inlet 84 is provided with a small-diameter portion 76 of comparatively small diameter on the high-pressure side facing the high-pressure chamber 71, and a large-diameter portion 77 of comparatively large diameter on the low-pressure side facing the exterior of the case 85. A shoulder portion 78 is formed between the small-diameter portion 76 and the large-diameter portion 77, and a female thread is formed on the inner circumference surface of the small-diameter portion 76. An annular groove 79 is disposed in the shoulder portion 78 to accommodate an O-ring 88.
The special thread member 87 inserted into the gas charge inlet 84 has a large-diameter portion 81, which is inserted into the large-diameter portion 77 of the gas charge inlet 84, and a small-diameter portion 80, which has a thread around its outer cylindrical surface which engages the female thread of the small-diameter portion 76, and the large-diameter portion 81 inserted into the gas charge inlet 84 presses on the O-ring 88 and seals the gas charge inlet 84.
The perimeter portion of the diaphragm 86 is sealed and supported on the outer perimeter portion of the case 85 by a weld portion 82 made by an electron beam or the like. In addition a saucer-shaped plate 89 is disposed on the diaphragm 86 as a stopper to define the limit of deformation of the diaphragm 86, and the plate 89 is also fastened around its circumference by the weld portion 82. A recess portion 83 shaped like one side of a convex lens is formed on the inner face of the plate 89, which gradually deepens from the outer perimeter portion of the diaphragm 86 towards the center, and communicating holes 90 are formed as fuel channels which communicate with the recess portion 83.
The case 85, the metal diaphragm 86, and the plate 89 are all hermetically sealed and bonded to each other around their outer perimeter portions by an electron beam, or the like. The space sealed between the metal diaphragm 86 and the case 85 is charged with a high-pressure gas such as nitrogen.
In the high-pressure fuel pump assembly 200 in FIG. 11, a male thread 91 formed around the outside of the case 85 engages a corresponding female thread formed in the recess portion 40c, and the high-pressure accumulator 70 is inserted into the plate 89, sealed by an O-ring 51, and secured to the recess portion 40c so as to allow the communicating holes 90 to communicate with the discharge passage 4b. 
The high-pressure accumulator 70 constructed in this way, absorbs surges in the pressure of the fuel discharged by the discharge passage 4b. That is, while fuel is being discharged through the discharge passage 4b, surges occur in the discharge passage 4b, for example, when the high-pressure fuel pump 3 is operating. The volume of the high-pressure chamber 71 varies in response to changes caused by the surges until the pressure of the high-pressure gas in the high-pressure chamber 71 reaches equilibrium with the pressure in the discharge passage 4b through the diaphragm 86. For example, when the pressure in the discharge passage 4b rises, the diaphragm 86 is deformed such that the volume of the high-pressure chamber 71 decreases and the volume of the discharge passage 4b increases, and so the pressure in the discharge passage 4b decreases and surging is reduced.
After charging the device with high-pressure gas such as nitrogen through the gas charge inlet 84, the O-ring 88 is inserted, the special thread member 87, which has a male thread portion, is screwed in, and the space between the case 85 and the special thread member 87 is sealed by the O-ring 88, sealing the high-pressure gas into the high-pressure accumulator 70.
However, the high-pressure accumulator 70 arranged in the manner described above suffers from the following problems:
because the gas charge inlet 84 is sealed in only one place, deterioration of the O-ring 88 can result in the high-pressure gas leaking from the high-pressure accumulator 70, leading to a decline in its ability to absorb surges;
sealing high-pressure gas into the high-pressure accumulator 70 is not easy and requires special equipment to work at atmospheric pressure;
because O-rings 88 are used, manufacturing costs are high; and
the gas charge inlet 84 must be a prescribed thickness to ensure enough thread to withstand the high pressure, but because it is disposed in the center of the high-pressure accumulator 70, it makes the case 85 thicker and therefore the size of the case 85 cannot be reduced.
Consequently, an object of the present invention is to provide a highly-reliable sealing device of simple construction for the gas charge inlet of a high-pressure vessel to overcome the above problems.
The sealing device according to the present invention is provided with a mechanical seal portion disposed in a gas charge inlet, and a welded seal portion disposed in the gas charge inlet on the low-pressure side of the mechanical seal portion.
Also, the sealing device according to the present invention is characterized in that the mechanical seal portion may be a steel ball pressed into the gas charge inlet to form a seal, and the welded seal portion may be a plug member disposed in the gas charge inlet on the low-pressure side of the steel ball and sealed by welding to the high-pressure vessel.
Also, the sealing device according to the present invention is characterized in that the mechanical seal portion and the welded seal portion may be formed in one member.
Also, the sealing device according to the present invention is characterized in that the one member may be a plug member provided with a press-fit portion pressed into the gas charge inlet and a welded seal portion disposed in the gas charge inlet on the low-pressure side of the press-fit portion and sealed by welding to the high-pressure vessel.
Also, the sealing device according to the present invention is characterized in that the one member may be a hollow plug member disposed in the gas charge inlet having a closed end on the high-pressure side and an open end on the low-pressure side, and in that it may be provided with a steel ball which is pressed inside the plug member to press the plug member against the inside of the gas charge inlet and form a seal.
Also, the sealing device according to the present invention may be provided with a plug member which has a tapered surface with its narrow end towards the high-pressure side, the tapered surface being ring-projection welded around its circumference to the gas charge inlet of the high-pressure vessel.
Also, the sealing device according to the present invention may be provided with a small-diameter portion disposed in the gas charge inlet, a female thread disposed on the low-pressure side of the small-diameter portion, a steel ball in contact with the small-diameter portion, and a plug member which engages the female thread in the gas charge inlet, and presses the steel ball against the small-diameter portion to form a seal.
Also, the sealing device according to the present invention is characterized in that the plug member may be provided with a weld portion around its circumference on the low-pressure side.
Also, the sealing device according to the present invention is characterized in that the gas charge inlet may be disposed on a perimeter portion of the high-pressure vessel.
Also, the sealing device according to the present invention is characterized in that the high-pressure vessel may be mounted on a high-pressure fuel pump assembly used in a cylinder-injected engine.